1. Field of the Invention
The present invention relates to a spot facing method and apparatus for processing a printed circuit board and, more particularly, to a spot facing method and apparatus suitable for achieving high dimensional precision in a depthwise direction.
2. Description of the Related Art
In recent years, there is a trend for multi-layered printed circuit boards to cope with demands for higher densities of packaging of wirings and components. The multi-layered structure essentially requires boring of holes which partly penetrate the multi-layered structure to provide electrical connections between adjacent layers, as well as spot facing in order to form recesses where electronic parts such as ICs are to be placed. Such boring and spot facing require a very high dimensional precision in the depthwise direction of, for example not greater than .+-. 0.05 mm.
Japanese Patent Unexamined Publication No. 61-131804 discloses a boring or spot facing method which can meet such a requirement. According to this method, it is possible to accurately form a hole down to a circuit on an internal layer, provided that a portion of the circuit is exposed at one end surface of the printed circuit board.
This method, however, cannot be used when the internal circuit is not exposed at the end surface of the printed circuit board or when the processing has to be done to an intermediate portion of an insulating layer.
The printed circuit board, which is composed of a plurality of layers of different materials bonded together, generally exhibits undulation or unevenness in thickness. In addition, since the position of a tool with respect to the spindle varies according to the type of the tool, it is impossible to conduct the processing by previously setting the amount of movement of the tool when a high processing accuracy is required.